(1) Field
The disclosed technology relates to a toner supply device, image forming apparatus and toner supplying method, in particular relating to an image forming apparatus that performs image formation with toner as well as a toner supply device and toner supplying method for use in the image forming apparatus.
(2) Description of the Prior Art
Recently, there have been increased demands for image forming apparatuses capable of high-speed operations, and as the number of printing (per unit time) increases the speed of the paper to be conveyed has been also enhanced. For example, conventionally the processing ability of an image forming apparatus with not lower than 60 sheets per minute (A4 short-edge feed) was previously regarded as a high-speed machines, but nowadays, the situation has changed and the machines having a processing speed of 80 sheets per minute or greater should be regarded as high-speed ones, and further, machines having a speed of 100 sheets per minute are being developed.
Since a large amount of toner is consumed in such image forming apparatuses, most of the developing units use a technology for keeping the toner concentration in the developing hopper constant and avoiding indication of “toner empty” from occurring when a large volume of printing has been performed. That is, the developing unit includes a plurality of toner containers arranged, and the toner supplied from each toner container is not directly fed to the developing hopper but is once collected in a toner feed device that functions as a “relay box”, then is fed into the developing hopper as the toner concentration therein becomes lower (see patent document 1: Japanese Patent Application Laid-open Hei 03-220577).
Even in such a toner supply arrangement where a plurality of toner containers are provided, it is necessary to avoid a lack of toner to be supplied to the developing unit. However, when, in the conventional toner supply system, one toner container is replaced, the operating time of the developing unit is limited by the amount of toner left in the toner container that is currently being used. Accordingly, the operating time is demanded to be long enough to afford plenty of time up to the replacement of the toner container.
However, in the conventional toner supplying method, when all the toner in the toner container being used was used up and hence so-called toner empty was detected, and if high-speed printing is being continuously implemented with another toner container, to be used second, which holds a lower amount of toner left therein, the time up to replacement of the toner container becomes short, so that there is the problem that the image forming apparatus becomes liable to stop due to a lack of toner supply.